< Cell >
Cell is the basic structural and functional unit of all known living organisms.
In the cell, the nucleus is separated from the cytoplasm by the nuclear envelope.
The nucleus contains chromosomes which are the genetic material of an organism. Every chromosome is composed of very long deoxyribonucleic acid (DNA). DNA is the gene which decides most biological traits such as eye color, blood type, or risk for specific diseases.
Human cell has 23 pairs of chromosomes (including 1 pair of sex chromosomes) (Fig. 8-11), giving a total of 46 (Fig. 8-5).
Fig. 16-5. Structure of DNA.
The shape of DNA is double helix with the attached bases. There are four kinds of bases: adenine (A), thymine (T), guanine (G), cytosine (C). A Hollywood film dealing with the DNA was titled “GATTACA.”
The order of sequential bases eventually decides protein in the cytoplasm.
Fig. 16-7. Transcription in nucleus, translation in cytoplasm.
In the cell nucleus, DNA makes the messenger ribonucleic acid (messenger RNA) which corresponds to a part of DNA. This process is called transcription.
The messenger RNA gets out of the nucleus and meets the equivalent transfer RNAs. The transfer RNAs, carrying the amino acids, induce the combination of the amino acids to produce protein. This process is called translation; the transfer RNA is like a dictionary for the translation. The translation is performed inside the ribosome of the cytoplasm.
The resultant protein is called gene expression because the protein results in the biological traits.
The produced protein is transferred by the Golgi complex. The Golgi complex is also involved in other material transport.
As another thing in the cytoplasm, the lysosome contains enzyme that can break down virtually all kinds of biomolecules. The lysosome acts as the waste disposal system (Fig. 12-4).
By breaking glucose down, the mitochondria (singular, mitochondrion) generate adenosine triphosphate (ATP) which is the currency of intracellular energy. The mitochondria are like power plant in the cytoplasm.
The basic function of the cell membrane is to separate the interior of cell from its surroundings. The cell membrane, a semipermeable membrane, controls the pass of substances such as protein (Fig. 16-7), ATP (Fig. 16-11) in and out of cells.
< Tissue >
Tissue is an ensemble of similar cells that together carry out a specific function.
Tissues are basically grouped into the epithelial, connective, muscle, and nerve tissues.
The epithelial tissue covers surfaces or lines the cavities throughout the body.
Functions of the epithelial tissue include protection, secretion, selective absorption, and sensing. Epithelial tissue contains no blood vessels, so it must receive nourishment via diffusion of substances from the underlying connective tissue proper (Fig. 16-23).
In the epithelial tissue, cells are so tightly packed that there is almost no ground substance (Fig. 16-29).
Fig. 16-17. Simiple (left) and stratified (right) epithelia.
Epithelial tissue is classified by the number of cell layers: Simple epithelium has only one cell layer, while stratified epithelium has two or more cell layers.
Fig. 16-18. Pseudostratified epithelium.
Pseudostratified epithelium looks like the simple epithelium since all the cells rest on the base. Simultaneously, this looks like the stratified epithelium since the short cells do not reach the surface.
Fig. 16-19. Squamous (ceft), cuboidal (center), and columnar (right) epithelia.
Epithelial tissue is also classified by the shape of the surface cells: squamous epithelium, cuboidal epithelium, columnar epithelium.
Fig. 16-20. Transitional epithelium.
Transitional epithelium has the surface cells that can change from cuboidal to squamous, depending on the expanded extent of the epithelium.
As the combination, the following epithelia are found in the individual organs.
The simple squamous epithelium is in the blood vessel (Fig. 10-77) and alveolus (Fig. 5-25) where something should pass the epithelium very easily. The simple cuboid epithelium is in the renal tubule (Fig. 6-11) where something should pass the epithelium adequately. The simple columnar epithelium is in the gastrointestinal tract (Fig. 4-45) where something should pass the epithelium and should not injure the epithelium.
The stratified squamous epithelium is in the skin (Fig. 15-2), oral cavity (Fig. 4-14) and vagina (Fig. 7-37) where something should not injure the epithelium.
The pseudostratified columnar epithelium is in the bronchus (Fig. 5-24) where something (mucus) coats on the epithelium. The transitional epithelium is in the urinary bladder (Fig. 6-18) where something (urine) makes the epithelium expand.
“Connective” tissue “connects” the different types of tissues in the body. Connective tissue is divided into the connective tissue proper and special connective tissue.
The epithelial tissue is supported by the connective tissue proper with no exception (Fig. 15-1).
Parenchyma is the cells that perform the specific function of the tissue or organ. On the other hand, stroma is the cells that support the parenchyma. Generally, the connective tissue proper plays the stroma role.
Fig. 16-25. Loose connective tissue.
The connective tissue proper is categorized into the loose connective tissue (e.g., subcutaneous tissue) with few extracellular fibers (Fig. 15-9) and the dense connective tissue. The dense connective tissue is durable because of the abundant extracellular fibers (Fig. 16-26). Likewise, the fibrous joint has durable fibrous tissue (Fig. 2-2). The fibrous membrane is more durable than the synovial membrane (Fig. 2-32).
Fig. 16-26. Dense irregular (left) and dense regular (right) connective tissue.
According to the direction pattern of fibers, the dense connective tissue is subdivided into the dense irregular connective tissue (e.g., dermis) (Fig. 15-18), and dense regular connective tissue (e.g., ligament, tendon) (Fig. 2-38) (Fig. 3-3).
The special connective tissue means the cartilage, bone, and blood.
All the connective tissue is composed of cells, ground substance, and fibers (Fig. 16-29). The cells of connective tissue proper, cartilage, bone, and blood (Fig. 1-3) tend to be round unlike the cells of epithelium (Fig. 16-17), muscle (Fig. 16-31), and nerve (Fig. 16-32).
Fig. 16-29. Composition of four tissues.
The cartilage and bone cells (chondrocyte and osteocyte) produce ground substance, which firmly surround the cells. In the case of bone, the main component of ground substance is calcium. Unusually, ground substance of the blood is fluid (plasma) (Fig. 11-17); fiber of the blood is fibrin that participates in clotting (Fig. 11-13).
Skeletal muscle is the voluntary muscle controlled by the somatic motor nerve (Fig. 13), while cardiac muscle and smooth muscle are the involuntary muscle controlled by the visceral motor nerve (autonomic nerve) (Fig. 13-60).
The skeletal and cardiac muscle cells have the striations in contrast with smooth muscle cell which does not have.
The nerve tissue is found in the central and peripheral nervous systems (Fig. 13-1).
An organ is formed by the multiple tissues in order to carry out the various functions.
Tissues are intermediate between cells and organs. In cytology class, we look at cell organelles few nanometers big; in histology class, we look at tissues few micrometers big; in anatomy class, we look at organs few millimeters big.